Objectives To understand the neural bases of visual deficits that occur during aging and to use the primate visual system as a model system for answering general questions about the effects of aging on the brain. ABSTRACT:Neuroanatomical studies carried out during the current year showed that there are no significant differences between young adult and old rhesus monkey retinae in the total number of ganglion cells, in the number of ganglion cells in particular retinal locations, or in ganglion-cell soma sizes. Comparisons between mean totals of LGN neurons and their input retinal ganglion cells indicated that there is about one ganglion cell for each LGN neuron. However, among the individual animals examined, there was no significant correlation between the numbers of these cells. Quantitative neuroanatomical measures in the rhesus monkey striate cortex suggest that there are no significant differences between young adult and old animals in areal density of cytochrome oxidase blobs. Neuron soma sizes also did not differ significantly between young adult and old animals or between blob and interblob regions. In addition, neuron density was not significantly different between young adult and old animals. However, there were significantly higher densities of neurons in interblobs than in blobs. Together with our previous studies, these results suggest that the peripheral parts of the visual pathways are relatively unaffected by aging in the rhesus monkey. We also carried out noninvasive electrophysiological studies in the rhesus monkey visual system to assess visual function of individual animals. Preliminary results for both young adult and old animals indicate that swept spatial frequency visual evoked cortical potentials (VECP) and pattern electroretinogram (PERG) estimates of spatial acuities are in close agreement. Both PERG and VECP estimates of spatial acuity are reduced in old animals in good ocular health, although there is some overlap with acuity values in the young adult animals. To complement the sweep-VEP method, we have begun training monkeys so that we can carry out behavioral psychophysical tests of vision. Keywords vision, visual cortex, retina, lateral geniculate nucleus, visual evoked cortical potentials, pattern electroretinogram, visual psychophysics, quantitative neuroanatomy